1. Field of the Invention
This invention relates to apparatus and methods for storing data on disks. In particular, this invention relates to high density data storage on floppy disks.
2. Related Applications
This application is also related to the following copending U.S. Patent Applications assigned to the assignee of the present inventions: "Mechansim For Clamping And Spinning A Flexible Disk", Ser. No. 428,936, filed Sept. 30, 1982, now abandoned; "A Mechanism For Clamping and Spinning A Flexible Disk", Ser. No. 428,944, filed Sept. 30, 1982; and "Flexible Magnetic Disk Clamping An Ejector Mechanism" Ser. No. 495,801, filed May 18, 1983, now U.S. Pat. No. 4,539,614.
3. Description of the Prior Art
A present "state of art" flexible or floppy disk drive records data on a series of concentric tracks on a rotated flexible magnetic disk. A transducer or magnetic read/write head is positioned near a desired track by an open loop stepper motor which has a predetermined increment of motion equal to one track per step. Conventional floppy disks have either 48 or 96 tracks per inch of disk radius. The motor is stepped with serial step pulses on command of the disk drive control unit, which maintains a count of the track position. When the head is moved to another track, the control unit will issue to the disk drive a polarity signal for direction and appropriate numbers and serial step pulses for distance. If the head is not properly aligned to a track, the head will read signals from the adjacent track, or from the previously written track, which will appear as noise to the desired signal. If the noise-to-signal ratio is large, the desired signal will be in error. There are three important factors which cause the head to be misaligned to the tracks:
1. the flexible disk is removable and upon reinsertion an eccentric misclamping error may occur;
2. the flexible disk expands and contracts in a non-uniform manner due to humidity and temperature variations; and
3. the stepper motor and actuator have inaccuracies that misalign the head.
To overcome these problems, it becomes necessary to "follow" the tracks with the head. When the track density is small, the head can be equipped with a trim erase element which can erase a clean zone on either side of the track. As the track densities are increased, the misalignment error may approach or become larger than the track spacing. The noise-to-signal ratio then becomes very large and the desired signal cannot be read. Additionally, trim erase elements will erase a portion of the adjacent track causing a further increase in the noise-to-signal ratio.
If the trim erase elements are removed from the head as a cost effective measure, it is necessary that the head-to-track error be no greater than plus or minus 0.0005 inches to assure favorable noise to signal ratio. A position error feedback to the head carriage actuator is required if the head is to follow the track with this accuracy. The device used to perform this function is commonly called a closed loop servomechanism. A conventional closed loop servomechanism requires both a continuous, off-track error signal and a velocity feedback signal. The head-to-track position error is continuously being reduced towards zero in an analog or continuous manner with a control velocity profile. The continuous source of the error signal is derived from dedicated servo tracks and the velocity feedback requires the use of a separate velocity transducer. This method is inefficient because a large portion of the data capacity of the drive is used to provide the error signal, and the velocity transducer adds cost and increases servo control complexity.
Various types of head positioners are known. A screw stepper has one stepper motor with a screw that is threaded through a follower. The follower is mounted on a carriage for driving the head inwardly and outwardly with respect to the lead screw. Advantages of the screw stepper include accuracy, good wear life, and non-accumulating error at a reasonable cost. A disadvantage is the possible incremental nut wear that can reduce positioning accuracy. A band stepper has a motor, capsan and a band trained about the capsan for moving the head carriage linearly with respect to the stationary frame. Band steppers are economical and fast, but have poor accuracy, and a tendancy for instable and noisy operation. A micro stepper has an electric motor that achieves fine steps by adjusting the current electronically so that the rotor moves in fine steps between the pole faces. Such a motor requires complex electronics and each motor has to be calibrated individually.
Various transducer positioning systems utilizing servo data in an embedded format are known. Coarse and fine radial positioning of a transducer relative to the concentric tracks of a rotatable magnetic disk having work data and servo data interspersed in alternate sectors on the disk is disclosed in U.S. Pat. Nos. 4,032,984 to Kaser et al., 4,149,200 to Card, and 4,149,201 to Card. Another example of a transducer positioning system for a rotating magnetic disk having servo positioning information interspersed with data is disclosed in U.S. Pat. No. 4,208,679 to Hertrich. A positioning system which provides a series of adjacent servo tracks, the boundary between adjacent servo tracks defining a path for the servo system to follow is shown in U.S. Pat. No. 3,691,543 to Mueller. U.S. Pat. No. 4,204,234 to Noble discloses a track following system wherein recorded data is successively sampled by a read/write head that is caused to move back and forth by a servo system towards that path defined by the maximum amplitude data signals.